A non-contact method of measuring the 3D shape of an object surface using a diffraction pattern of laser light from a diffraction grating is generally known (for example, patent literature (PTL) 1). In this method, a line-shaped diffraction pattern is projected onto a measured object surface, and the projected image is observed from a predetermined position with an imaging device, such as a solid-state imaging device. Since the line-shaped pattern that is observed changes shape in accordance with the unevenly shaped surface of the measured object, the 3D shape of the surface to be measured is calculated by analyzing the imaged pattern.
For example, PTL 1 discloses a measurement apparatus disposed at the tip of an endoscope. In this measurement apparatus, a laser beam for interference fringe projection is guided using a polarization-maintaining fiber. Two laser beams of linearly polarized light, having mutually orthogonal polarization directions, are caused to be incident at the incident side of the polarization-maintaining fiber, so that the respective polarization directions become the polarization-maintaining directions of the polarization-maintaining fiber. A birefringent plate is placed at the exit-side of the polarization-maintaining fiber so that one of the polarization-maintaining directions of the polarization-maintaining fiber matches the direction of the optical axis. As a result, the light beam emitted from the polarization-maintaining fiber is separated in two. Furthermore, by providing a polarizing plate with a transmission axis inclined 45° relative to the optical axis of the birefringent plate, only the coherent components are extracted from among the two polarization components and projected onto the measured object to generate an interference fringe. According to PTL 1, a mechanism to adjust the phase difference of the two laser beams of linearly polarized light is provided at the light source side of the polarization-maintaining fiber, and the interference fringe projected on the measured object can be scanned. As a result, the shape of the measured object is calculated and output on the basis of the principle of fringe scanning.